Energy input-output analysis of paddy fields under different irrigation modes and nitrogen fertilizer managements

Coupled water-nitrogen management has the potential to improve rice productivity while reducing energy demand, yet its integrated effects on the energy balance of cold-region rice production systems remain insufficiently quantified. To address this issue, a two-year field experiment was conducted in Northeast China from 2021 to 2022 to evaluate the interactive effects of irrigation regime and nitrogen (N) rate on rice yield and energy dynamics. The experiment adopted a factorial design with two irrigation modes, flood irrigation (F) and controlled irrigation (C), and four N application rates (0, 85, 110, and 135 kg N ha^-1), resulting in eight treatments with three replicates each. Energy input (EI), energy output (EO), net energy (NE), and energy use efficiency (EUE) were quantified for all treatments. The results showed that the highest N rate (135 kg N ha^-1) produced the greatest EI across treatments and increased dependence on non-renewable energy, while diesel, machinery, and fertilizers together accounted for more than 73% of total EI. Under controlled irrigation, C-N110 achieved the highest EO (292,046 MJ ha^-1) and NE (252,425.25 MJ ha^-1), and its grain yield was significantly higher than that of the other treatments (P < 0.05), indicating that improved energy performance was accompanied by enhanced productivity. Under flood irrigation, F-N110 reduced EI compared with F-N135 while maintaining comparable EO, suggesting superior energy performance under a stable water supply. Overall, these findings demonstrate that both irrigation regime and N rate strongly affect the energy performance of rice production in cold regions, and that controlled irrigation combined with a moderate N rate of 110 kg ha^-1 is an effective strategy for improving both rice yield and energy efficiency.